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• If the session partner is not on an adjacent node, the network node
server for the LU requesting the session uses the network topology
database to identify network nodes and intermediate transmission
groups in the route to the session partner.
• If the session partner is on an end node, the end node (or its
network node server) provides information about the link between
the network node server and that end node (or the link between
the end node and a connection network).
3. Excludes all network nodes and transmission groups that do not meet
the specified characteristics for the session route.
4. Computes the optimal route for the session.
Depending on the specified class of service, the route calculation
algorithm computes a weight value for each node and logical link and
then totals the weights for each route. To select the optimal path, the
network node computes the current least-weight route from the node
containing the originating LU to the node containing the destination LU.
Intermediate Routing
Intermediate routing enables an APPN network node to receive and
route data destined for another node. The origin and destination of the
data can be an end node, another network node, or a LEN.
Intermediate routing supports sessions between LUs that are not on
adjacent nodes. After a route has been selected for a session, APPN
network nodes in the route use intermediate routing to forward session
data to the next node in the route.
Resource characteristics maintained by the topology database can
include congestion status. If a network node becomes heavily congested,
the network node can relay this information to other network nodes in
the network, making the congested network node less likely to be
included in session routes calculated for new sessions.
APPN provides two types of intermediate routing:
• In intermediate session routing (ISR), available in all network nodes,
the network node keeps track of each intermediate session. Each
intermediate node adjusts the pacing of session data to control the
rate at which data flows between adjacent nodes. Each intermediate
node can also perform segmentation and reassembly of segmented
Chapter 1
SNA Terms and Concepts
Basic APPN Concepts
57
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